The present invention relates to a method of producing microcapsules each having a wall of a polyurethane-urea resin film.
For producing microcapsules by adding a polyisocyanate to an oily liquid containing a capsule core substance, emulsifying and dispersing the oily liquid and an aqueous liquid containing an emulsifier to form an emulsion, and reacting a polyamine with the emulsion to form a synthetic polymer wall film at the liquid-drop interface to encapsulate the hydrophobic liquid surface; the following methods are known.
(1) A method in which the reaction is initiated in a high stirring mixing zone to form an emulsion, then the reaction product is transferred to a multi-stage cascade of a container, and the temperature is stepwise elevated and the temperature in each step is kept constant [JP-A-SHO.55-124534 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")].
(2) A method in which a polyisocyanate solution is mixed and dissolved in an oily liquid containing a capsule core substance and thereafter the solution is mixed with an aqueous liquid containing an emulsifier to be fed to an emulsifying device for continuous emulsification (JP-A-SHO.56-108529).
(3) A method in which the emulsified dispersion is continuously passed through a tubular reactor and is reacted therein at a temperature of from 40.degree. to 95.degree. C. (JP-A-SHO.58-112042).
(4) A method in which a polyisocyanate insoluble in the oily liquid is mixed in an inert organic phase, the mixture is continuously emulsified in an aqueous phase, and a diamine or polyamine is added to the resulting emulsion for poly-addition to the interface with the polyisocyanate (JP-A-SHO.61-21728).
(5) A method in which the dispersion is heated by direct introduction of steam thereinto for encapsulating the hydrophobic oily liquid film with the synthetic polymer wall (JP-A-HEI.2-139030).
One example of a typical conventional production method is explained with reference to the flow-sheet of a production apparatus of FIG. 2, in which an oily liquid containing a capsule core substance is prepared at a temperature of 80.degree. C. or higher, the oily liquid is cooled to room temperature in the tank 1, an aqueous liquid formed by dissolving polyvinyl alcohol in water at 80.degree. C. or higher is cooled to room temperature in the aqueous liquid tank 6, a polyisocyanate is prepared in the tanks 2 and 3, a polyol is prepared in the tank 4, a polyisocyanate and a polyol each of a determined amount are continuously added via the respective pumps 1a, 2a, 3a and 4a, the mixture liquid is continuously added to the aqueous liquid as fed via the metering pump 6a with mixing it in the in-line mixer 5, to conduct emulsification and dispersion of the mixture liquid through the preemulsifier 7 and the emulsifier 8, the resulting emulsion is transferred to the encapsulating tank 9, a polyamine and a concentration adjusting water are added to the encapsulating tank 9 from the polyamine tank 10 and the concentration adjusting water tank 11, respectively, via the respective pumps 10a and 11a, when a suitable amount of the emulsion has gathered in the tank 9, and the content in the tank 9 is heated up to 80.degree. C. with a tank jacket with stirring and is kept at the temperature for 60 minutes to obtain a capsule liquid.
However, in all the conventional methods (1) to (5), emulsification is effected at room temperature so as to prevent encapsulation reaction during emulsification and thereafter the reaction system is heated for effecting wall filming treatment of the system. Therefore, these involve the following problems.
Since the polyisocyanate to be used for formation of the polyurethane wall is highly reactive, microcapsules having poor heat resistance and solvent resistance are formed if the temperature for treatment of the encapsulating system is not elevated immediately after emulsification. For elevation of the temperature, there have been proposed a method of stepwise elevating the temperature of the emulsion of (1) with a jacket or a hose line after the emulsion has been put in a tank (JP-A-SHO.55-124534); and a method of directly introducing steam in the emulsion of (5) (JP-A-HEI.2-139030). However, both of them need much time for elevating the temperature so that a uniform wall film could not be formed and, as a result, the heat-resistance and the solvent resistance of microcapsules formed are not sufficient.
The oily liquid and the aqueous liquid are prepared at a high temperature of 80.degree. C. or higher. However, where emulsification of them is effected at a low temperature, they are once cooled and then emulsified and thereafter are again heated for encapsulation reaction. The operation involves much energy loss. In addition, where the oily liquid contains a colorant or the like as the capsule core substance, the colorant would easily precipitate when it is cooled so that the concentration of the colorant in the liquid could not be made high defectively.
Further, since the encapsulation is effected in the tank to which the liquids are added thereto from the top thereof, capsules formed adhere to the vapor-liquid interface of the inner wall of the tank. The adhered capsules must be removed periodically by cleaning, which needs intermittence of the production process to result in depression of the production yield and elevation of the production cost. As a countermeasure to the problem, a method of continuously introducing the emulsion to the tubular reactor for effecting the encapsulation reaction is proposed in JP-A-SHO.58-112042 of (3). However, the method still involves various problems that aggregated capsules are formed because of the insufficient stirring in the reactor, that a large-sized equipment is necessary for ensuring a determined retention time for encapsulation and that removal of the capsules as adhered to the wall in the reactor is difficult.